It is known that the flammability of polymers and plastics can be reduced by the addition of fire retarding substances. It is also known that triazines, and triazine compounds, such as melamine cyanurates and melamine, are fire retarding substances.
Dietrich et al in U.S. Pat. No. 3,660,344 disclosed a self-extinguishing polyamide moulding composition comprising a polyamide containing between 0.5 and 25 percent melamine, or melamine derivatives.
This concept was expanded upon in U.S. Pat. No. 4,001,177 by Tsutsami who disclosed a flame retarding polyamide composition containing between 3 and 40 percent melamine, and between 0.5 and 20 percent cyanuric acid, isocyanuric acid, or similar cyanurates. The cyanurates were said to improve the distribution of melamine within the polyamide.
The flame retardancy technique with triazines was again advanced by Ohshita and Tsutsami in U.S. Pat. Nos. 4,321,189 and 4,363,890, disclosing a flame retardant polyamide molding resin containing melamine cyanurate finely dispersed therein, and the process for its preparation. In their teachings it was necessary that the average crystal size of the melamine cyanurate finely dispersed in the polyamide be less than 250 angstroms, or 0.0250 micron. To achieve the extremely fine melamine cyanurate, Ohshita and Tsutsami separately melt blended melamine and cyanuric acid with polyamide and then melt blended both polyamide blends with each other thereby producing melamine cyanurate. In an alternate preparation method, they melt blended equimolar amounts of melamine and cyanuric acid in polyamide containing between 1 and 30 percent water. Although melamine cyanurate in polyamide prepared by Ohshita and Tsutsami was effective as a fire retardant in relatively low concentrations of melamine cyanurate, the extremely high surface area and low bulk density makes molten polyamides containing useful amounts of melamine cyanurate extremely viscous and limits the amount of melamine cyanurates which can be effectively used to a maximum of about 30 percent. Higher concentrations downgrade the mechanical properties of the polyamides.
Kawasaki et.al. in U.S. Pat. No. 4,317,766 prepared flame retarding melamine cyanurate with particle sizes of about 10 microns in a more difficult manner by putting the melamine and cyanuric acid in the monomer mixture and them polymerizing to form polyamides. Although they were able to use the more reasonable sized particles of about 10 microns, they were still limited to between 2 and 25 percent melamine cyanurate concentrations. Kawasaki et.al. point out that the addition of cyanuric acid in aqueous solution is not practical because of its very low water solubility in water.
Although the prior art has provided teachings on the use of triazine compounds, particularly melamine and melamine cyanurate as fire retardants in polyamide resins, little information has been provided which would lead to a highly desirable, and much needed, cyanurate adduct fire retardant concentrate for providing fire retardancy to thermoplastic polymers by easy melt blending therewith. Also, no effective method for the preparation of such a concentrate has been provided or suggested.
The development of such a concentrate has been prevented by the inability to produce good physical properties in a resin based concentrate containing more than 25 to 40 percent cyanurate based fire retardant. This inability stems from the fact that solid particles are difficult to blend with thermoplastic polymers, and from the low solubilities of the cyanurates used in the prior art. These low solubilities have precluded use of any simple and economical cyanurates.
Thus, it has not been possible heretofore to precipitate extremely insoluble, fine, and dense cyanurate adducts homogeneously into fluid polymers in concentrated form from solution.
Direct introduction of fire retarding powders of melamine cyanurate, or other adducts, directly into fire retarding thermoplastic polymers by means of a mixing device such as a twin screw melt blender causes heavier wear on the blending device than particles of thermoplastic polymers, and more importantly requires a higher concentration of fire retardant because complete homogeneity is not achieved. The lack of homogeneity also seriously detracts from the physical properties of the polymer.
The prior art has brought forward no method for achieving the ultimate in integration of cyanurate adducts into thermoplastic fire retardant concentrates by using soluble cyanuric acids or cyanurates to achieve miscibility before precipitation as completely insoluble triazine cyanurate adducts to produce high concentrations of non-bleeding fire retardant in thermoplastic resins.